1. Field of the Invention
The present invention pertains to the field of thin films for use in ferroelectric integrated circuits, and, more particularly, electrodes for use in such ferroelectric integrated circuits.
2. Statement of the Problem
Ferroelectric compounds possess favorable characteristics for use in nonvolatile integrated circuit memories. See U.S. Pat. No. 5,046,043 issued Sep. 3, 1991 to Miller et al. A ferroelectric device, such as a capacitor, is useful in a nonvolatile memory when it possesses desired electronic characteristics, such as high residual polarization, good coercive field, high fatigue resistance, and low leakage current. Layered superlattice materials have been used in integrated circuits. U.S. Pat. No. 5,519,234 issued May 21, 1996, to Araujo et al., U.S. Pat. No. 5,434,102, issued Jul. 18, 1995, to Watanabe et. al., and U.S. Pat. No. 5,468,684, issued Nov. 21, 1995, to Yoshimori et al., describe these materials and processes for integrating these materials into practical integrated circuits. Perovskites, sometimes referred to as ABO3-type materials, such as PZT and PLZT compounds, have also been used in integrated circuits. See Franco Jona and G. Shirane, Ferroelectric Crystals, Chapter IV and V, pp. 109-261, Dover Publications Inc, New York, 1993, for a general discussion of ABO3-type materials.
Ferroelectric devices containing thin film ferroelectric materials are currently being manufactured. Some ferroelectric materials comprise bismuth among other elements. The desirable characteristics of a ferroelectric device, including high residual polarization, good coercive field, high fatigue resistance, and low leakage current, can be limited during manufacturing of the ferroelectric devices, due to the process temperatures conducted on the ferroelectric device during manufacturing. Annealing and baking temperatures can degrade the thin film ferroelectric material by diffusing chemical elements, necessary for good ferroelectric quality, from the thin film ferroelectric material to the electrodes.
As an example, when strontium bismuth titanate (SBT) is used in the thin film ferroelectric material, manufacturing temperatures can cause the bismuth to diffuse from the ferroelectric material to the adjacent electrodes. This causes a lower concentration of bismuth to exist in the ferroelectric material near the ferroelectric/electrode interface. It is critical to inhibit the diffusion of bismuth from the ferroelectric material to the electrodes because alteration of the amount of bismuth will change the ferroelectric properties. See U.S. Pat. No. 5,434,102 referenced above. Likewise, alteration of the amount of bismuth can create defects which alter how the ferroelectric responds to electrical fields. Similarly, diffusion or modification of other elements in the ferroelectric materials must be avoided if the ferroelectric materials and devices are to retain the properties specified.
Another problem is the relatively high manufacturing temperatures used in ferroelectric integrated circuits. The minimum feasible manufacturing process temperatures of ferroelectric materials used in the prior art is about 700xc2x0 C., which is the temperature at which deposited ferroelectric materials are annealed to achieve good crystallization. It is important that any improvement in the electrode composition be able to withstand these annealing temperatures.
For the above reasons, therefore, it would be useful to have ferroelectric devices and methods for fabricating ferroelectric devices in ferroelectric integrated circuits in which the diffusion of chemical elements from the ferroelectric material to the electrodes is inhibited. Further, any such electrodes should be capable of withstanding conventional annealing times and temperatures without becoming liquid.
A solution to the problem is to provide an electrode adjacent the thin film ferroelectric material with a chemical composition that comprises at least one of the metallic chemical elements of the ferroelectric material. This chemical composition of the electrode reduces the diffusion of chemical elements from the thin film ferroelectric material to the electrodes. By decreasing the diffusion of metal elements from the thin film ferroelectric material, the ferroelectric device electrical properties, such as the polarizability, are preserved over long periods, and the reliability of the integrated circuits is greatly improved.
The metal element of the ferroelectric material may exist in the electrode in the pure metal form, as an alloy, as part of a crystalline compound, or as part of an amorphous material. The electrodes may be formed by a single layer, or as multilayer structures, providing the layer adjacent the ferroelectric contains at least one of the metal elements of the ferroelectric.
The invention provides a ferroelectric integrated circuit including a substrate supporting a thin film ferroelectric material and an electrode layer in contact with the ferroelectric material, the ferroelectric material comprising a compound including a metal element, the electrode comprising the metal element. Preferably, the electrode consists essentially of the metal element. Preferably, the metal element is titanium. Preferably, the ferroelectric material comprises bismuth titanium oxide, bismuth titanate, or bismuth lanthanum titanate. Preferably, the electrode comprises a multi-layer structure including a first layer comprising the titanium and a second layer comprising platinum.
In another embodiment, the electrode comprises a metal alloy containing the metal element. Preferably, the alloy is a platinum bismuth alloy. Preferably, the bismuth concentration is 50% or less. Most preferably, the platinum concentration in the alloy is 60% or greater. Preferably, the metal alloy is eutectic. Preferably, the ferroelectric material comprises a layered superlattice material containing bismuth. Preferably, the ferroelectric material comprises either strontium bismuth tantalate or strontium bismuth tantalum niobate. Preferably, the alloy is a platinum titanium alloy. Preferably, the ferroelectric material comprises bismuth titanate, bismuth titanium oxide, or bismuth lanthanum titanate.
In a preferred embodiment, the thin film ferroelectric material comprises a superlattice generator metal and the metal element comprises the superlattice generator metal. Preferably, the superlattice generator metal is bismuth.
Preferably, the ferroelectric material comprises strontium bismuth tantalate or strontium bismuth tantalum niobate.
In still another embodiment, the electrode comprises a crystalline compound containing the metal element. Preferably, the crystalline compound comprises an oxide. Preferably, the oxide comprises a metal oxide.
In another embodiment, the electrode comprises an amorphous material containing the metal element. Preferably, the amorphous material is an amorphous oxide. Preferably, the amorphous oxide comprises an oxide of a metal selected from the group consisting of bismuth, titanium, and tantalum.
In a preferred embodiment, there are two of the electrodes and each of the electrodes comprises the metal element. Preferably, one of the electrodes comprises a multi-layered electrode, each of the layers comprising a different material, wherein one of the layers comprises the metal element. Preferably, both of the layers comprise the metal element. Preferably, both of the electrodes comprise a multi-layered electrode.
The invention also provides a method of making a ferroelectric integrated circuit, the method comprising the steps of: applying a first electrode layer comprising at least one of the chemical elements of a thin film ferroelectric material layer to a substrate; and forming the thin film ferroelectric material layer adjacent to the first electrode layer. Preferably, the method includes applying a second electrode layer comprising at least one of the chemical elements of the thin film ferroelectric material layer adjacent the thin film layered superlattice material layer. Preferably, one of the acts of applying or the step of forming comprises annealing.
The present invention also provides a fabrication method of the ferroelectric device electrodes that can withstand the conditions of conventional annealing processes while decreasing or inhibiting the diffusion of chemical elements from the ferroelectric material layer to the electrode layer of the integrated circuit. In addition, the method is compatible with conventional ferroelectric integrated circuit processes, and in most embodiments does not increase the number of processing steps; thus, it is economical to incorporate into the fabrication process. Numerous other features, objects and advantages of the invention will become apparent from the following description when read in conjunction with the accompanying drawings.